Structural and Bonding Principles in Metal Halide Cluster Chemistry

Abstract

ABSTRACT. Metal cluster halides M6X'2 and M'6Xa and their alkali metal derivatives exhibit a variety of structure types because of the evident need to bond terminal or shared halide at all metal vertices. The versatility of systems with group 3 and 4 transition metals is greatly increased by the presence of interstitial heteroatoms (Z) within all thermodynamically stable clusters. The 24 examples of Z provide a wide range of valence electrons and include many that are themselves metals. Structural and bonding principles are described, including the novel characteristics of some cation sites, the regular effects that cations have on cluster bond distances, matrix effects that arise because of characteristic X··X or M -X distances and their influence on metal -metal bonding and electronic configurations, and the use of metal -metal hnl'1d orders to characterize matrix effects. The condensation products of octahedral clusters, particu:arly chains centered by heterometals and double-metal sheet products, are also noted. Condensation to ClustersThe clustering of two to six (or more) metals within a nonmetal matrix is a frequent event among reduced compounds of the early transition (d) elements. The formation of such clusters is probably best known among the chalcogenides, while only a relatively few halide examples were known for Nb, Ta and Mo 10-15 years ago. The relative proportion of anions for a fixed oxidation state of a metal obviously increases in the series pnictide, chalcogenide, halide (e.g., Vp, V 2 S 3 , VCI 3 ), and so the number of possible metal-metal interactions decrease in parallel. Thus, highly reduced pnictides and chalcogenides often exhibit condensed, complex, and relatively isotropic structures, whereas halides more often show limited and lower dimensional interactions between the valence-active metals, i.e., in isolated clusters and chains. The latter characteristics not only provide very novel results but also make more evident many of the bonding and structural principles that go into the determination of cluster and condensed cluster structures. Much of the background to this area can be found in Schafer and Schnering (1964) and Wells (1984).The beginning of metal-metal bonding is often seen structurally whenever the stoichiometric number of nonmetals (halides) per metal in a reduced compound is less than the preferred coordination number so that coordination polyhedra must be shared. Thus, ZrI4 achieves CN6 by sharing two edges of each ZrI6 octahedron, viz, ZrI 2 1 4/2 . Only when valence electrons are available does this allow 27 E. Parthe (ed.).

Steric and Electrostatic Effects in Compounds with Centered Clusters Quantified by Bond Order Analysis

“…Thus, in this case, the Z atom may be regarded as an anion with the valence equal to 8 − k. In contrast, for Z represented by TM, the total number of bonding electrons is constant and equal to 7, testifying the presence of metal−metal M−Z interactions. This bonding picture is in contradiction with that suggested by Corbett 32 but agrees well with the MO diagrams proposed by Lin and Fan. 31 The study showed that, in spite of the high strains in the individual bonds, the total BVS of the M atoms was close to the number of its valence electrons (four for Zr, three for Sc, Y and Gd).…”

“…In contrast to the common consensus on the molecular orbital (MO) diagram for compounds with Z = main group atoms (Figure S1a), there are two different versions of such diagrams in the case of Z = TM (Figure S1b,c). , Both the versions give the same number of low-lying orbitals (nine) but differ by a relative position of two nonbonding e g orbitals. The MO diagram proposed by Lin and Fan was based on the interaction of the frontier orbitals of individual metal fragments derived from their local ML n coordination .…”

“…In contrast, according to Corbett, who used extended Huckel calculations, in such a compound like (FeZr 6 )I 18 , the energy of the nonbonding e g orbitals is lower than that of t 1u orbitals. 32 As a result, only four of eight electrons of Fe should be bonding. Following such a scheme, the BO of the M−Z bonds should be equal to 4/6 = 0.667 v.u.…”

Cluster Compounds with Oxidised, Hexanuclear [Nb6Cli12Ia6]n− Anions (n=2 or 3)

“…[ 52 , 53 , 54 , 55 , 56 , 57 , 58 ] An even smaller value of 2.9506 Å is found in [Nb 6 Cl 12 (PrCN) 6 ][SbCl 3 ] which can be explained by the much less pronounced matrix‐effect of the exo ‐N‐bonded propionitrile compared to the chloride ligands. [ 22 , 59 , 60 ]…”

“…[ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ] The large group of cluster compounds with octahedral metal atom polyhedra of the IVB and VB group metals are particularly in focus. [ 22 , 23 , 24 , 25 , 26 ] The M 6 octahedra are surrounded by a ligand sphere of either eight (for M =Nb and X =I; [ M 6 X 8 ] cluster type) or twelve “inner” ligands X i , ( X i =Cl or Br; [ M 6 X 12 ] type). In the [ M 6 X 8 ] cluster type, the X i ligands bridge the faces of the M 6 octahedron and in the [ M 6 X 12 ] type the edges.…”